1. Field of the Invention
The present invention relates to a surface-mount crystal oscillator, and more particularly, to a surface-mount crystal oscillator more suitable for miniaturization.
2. Description of the Related Arts
Surface-mount crystal oscillators each including a quartz crystal blank and an IC chip containing an oscillation circuit using the crystal blank, and having a structure suitable for surface mounting are widely used as reference sources of frequency and time in portable electronic equipment represented by, for example, portable telephones because of their compactness and light weight. As such a surface-mount crystal oscillator, Japanese Patent Laid-Open No. 2005-311769 (JP-A-2005-311769) discloses the one in which an IC chip and a crystal blank are integrated, and are hermetically encapsulated in a container. FIG. 1A is a cross-sectional view showing one example of a configuration of such a conventional surface-mount crystal oscillator, and FIG. 1B is a plan view of an IC chip used in the crystal oscillator.
The illustrated crystal oscillator includes container body 1 which is constituted of laminated ceramics, and has a flat rectangular parallelepiped shape, with a recess formed on one principal surface. IC chip 2 and crystal blank 3 are housed in the recess of container body 1, and hermetically encapsulated in the recess by covering the recess with metal cover 4.
IC chip 2 is in a substantially rectangular shape, in which electronic circuits including an oscillation circuit using crystal blank 3 are integrated on a semiconductor substrate. In the example shown here, a temperature compensating mechanism for compensating a frequency-temperature characteristic of crystal blank 2 as a quartz crystal element, and enhancing stability of oscillation frequency with respect to a temperature change is also integrated in IC chip 2. Accordingly, the crystal oscillator is configured as a temperature compensated crystal oscillator (TCXO).
The electronic circuits such as the oscillation circuit and the temperature compensating mechanism are formed on one principal surface of the semiconductor substrate in IC chip 2 by an ordinary semiconductor device fabricating process. Thus, out of both principal surfaces of IC chip 2, the surface on which the electronic circuits are formed in the semiconductor substrate will be called as a circuit formation surface. A plurality of IC terminals for connecting the electronic circuits in IC chip 2 to an external circuit are provided on the circuit formation surface as shown in FIG. 1B. These IC terminals include crystal connection terminals 5x, 5y which are used for electrical connection with crystal blank 3, and further include IC external terminals 5z which include at least a power supply terminal, an output terminal and a ground terminal. In FIG. 1B, the dashed line shows the disposed position of crystal blank 3. In the example shown here, IC external terminals 5z further include an AFC terminal used for input of an automatic frequency control (AFC) signal. Further, as one of the IC external terminals, write terminal 5z1 used for writing temperature compensation data to the temperature compensating mechanism is provided.
When the temperature compensation data can be supplied into IC chip 2 by using other IC external terminals 5z, write terminal 5z1 specially designed for writing the temperature compensation data is not formed.
IC external terminals 5z including write terminal 5z1 are connected to back surface terminals 15 formed on the other principal surface, that is, the principal surface which is not the circuit formation surface, of IC chip 2 via through-electrode 6 formed in IC chip 2. Crystal connection terminals 5x, 5y, IC external terminals 5z, 5z1 and back surface terminals 15 are all formed on the semiconductor substrate configuring IC chip 2 as square or rectangular electrode pads.
On an inner bottom surface of the recess of container body 1, circuit terminals (not shown) which are used for electrical connection with IC chip 2 are provided corresponding to the positions of back surface terminals 15 in IC chip 2. Back surface terminals 15 are electrically joined to the circuit terminals by using a so-called flip chip bonding technique such as ultrasonic thermo-compression bonding using bumps 7, whereby IC chip 2 is fixed to the inner bottom surface of the recess, and electrically and mechanically connected to container body 1.
A plurality of mounting terminals 8 which are used when the crystal oscillator is surface-mounted on a wiring board are provided as an electrode layer on an outer bottom surface of container body 1. Circuit terminals of the inner bottom surface of the recess are electrically connected to mounting terminals 8 via conductive paths formed in container body 1. Mounting terminals 8 include at least a ground terminal, a power supply terminal and an output terminal. In this example, mounting terminals further include an AFC terminal and a write terminal for temperature compensation data.
Crystal blank 3 which is used in such a crystal oscillator is constituted of, for example, a substantially rectangular AT-cut quartz crystal blank, and excitation electrodes 9a are formed on both principal surfaces of the crystal blank, as shown in FIG. 2. From a pair of excitation electrodes 9a, lead electrodes 9b are extended to both sides of one end portion of crystal blank 3. Lead electrode 9b is formed at a position of the end portion of crystal blank 3 to be folded back between both the principal planes of crystal blank 3. Crystal blank 3 is fixed to IC chip 2 and electrically connected to IC chip 2 by fixing lead electrodes 9b respectively to crystal connection terminals 5x, 5y on the circuit formation surface of IC chip 2 by, for example, conductive adhesive 10 at the position where a pair of lead electrodes 9b are led.
Metal cover 4 which is used for closing the recess of container body 1 is joined to an opening end surface of container body 1, that is, a surface which is a top surface of container body 1 and surrounds the recess, by seam welding or thermo-compression bonding. When seam welding is used, a metal ring is provided on the top surface of container body 1 along an outer perimeter of the recess, and metal cover 4 is joined to the metal ring. When thermo-compression bonding is used, joining is performed by using a eutectic alloy such as Au—Sn (gold-tin). In any case, metal film 11 is provided on the opening end surface of container body 1. Metal film 11 is electrically connected to the ground terminal out of mounting terminals 8 by a through-hole and the like (not shown) provided in a frame wall portion of container body 1, whereby metal cover 4 is electrically grounded.
A bottom wall of container body 1 has a structure in which a plurality of ceramic sheets are laminated, for example, and shield electrode layer 12 is formed on a lamination plane between these ceramic sheets. Shield electrode layer 12 is also electrically connected to the ground terminal of mounting terminals 8 by a through-hole or a via-hole formed in container body 1. Both metal cover 4 and shield electrode layer 12 are grounded, and thereby, IC chip 2 and crystal blank 3 are electrically shielded in both upper and lower directions.
However, in the surface-mount crystal oscillator of the above described configuration, since IC chip 2 and crystal blank 3 are integrated and housed in the recess of container body 1, the size of the crystal oscillator is determined by the height and the planar outer shape of container body 1 which do not directly relate to the actual function as the crystal oscillator. Accordingly, the planar outer shape and the height dimension of the surface-mount crystal oscillator cannot be made smaller than the size of container body 1 which can house the integrated IC chip 2 and crystal blank 3.